1. Field of the Invention
The present invention relates to an internal voltage generator, and more particularly to an internal voltage generator capable of outputting a back-bias voltage used as a substrate bias voltage of a semiconductor device.
2. Description of the Prior Art
Generally, an internal voltage generator is a circuit for generating an internal voltage used in a semiconductor device. The internal voltage generator includes a high voltage generator for generating an internal voltage larger than a supply voltage, a back-bias voltage generator for generating an internal voltage smaller than a ground voltage, a reference voltage generator for generating a predetermined reference voltage, etc.
For example, in a case of a memory device, which is a kind of semiconductor device, a high voltage generator is used for activating a word line. Further, a back-bias voltage generated by a back-bias voltage generator is applied to an N-well including a PMOS transistor, thereby adjusting the threshold voltage of the PMOS transistor.
FIG. 1 is a block diagram of a general internal voltage generator (in particular, a back-bias voltage generator).
As shown in FIG. 1, the back-bias voltage generator for generating a back-bias voltage VBB (internal voltage) includes a detection circuit 101 for detecting a back-bias voltage, a driving circuit 102 for outputting a predetermined pulse signal in response to the output signal of the detection circuit 101, and a pumping circuit 103 for lowering the back-bias voltage by a pumping operation when the pulse signal is applied from the driving circuit 102.
In the operation of the back-bias voltage generator, the pumping circuit 103 continues to perform the pumping operation until the back-bias voltage outputted from the pumping circuit 103 reaches a target level. Accordingly, after the back-bias voltage has reached the target level, the pumping circuit 103 stops the pumping operation and maintains the back-bias voltage at that time.
However, in the prior art, when a supply voltage VDD operating the driving circuit 102 of the back-bias voltage generator is unstable, the pulse signal applied to the pumping circuit 103 is also unstable. Therefore, it is difficult to generate a stable back-bias voltage.
In particular, since the supply voltage VDD of a semiconductor device has been recently lowered, the instability of the supply voltage may immediately cause the instability of an internal voltage.